# operator==(std::move_only_function)

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< cpp‎ | utility‎ | functional‎ | move only function

friend bool operator==( const std::move_only_function& f, std::nullptr_t ) noexcept; |  | (since C++23)

Checks whether the wrapper f has a callable target by formally comparing it with std::nullptr_t. Empty wrappers (that is, wrappers without a target) compare equal, non-empty functions compare non-equal.

This function is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when `std::move_only_function<FunctionType>` is an associated class of the arguments.

The `!=` operator is synthesized from `operator==`.

### Parameters

f | - | `std::move_only_function` to compare

### Return value

!f.

### Example

Run this code

    #include <functional>
    #include <iostream>
    #include <utility>

    using SomeVoidFunc = std::move_only_function<void(int) const>;

    class C {
    public:
        C() = default;
        C(SomeVoidFunc func) : void_func_(std::move(func)) {}

        void default_func(int i) const { std::cout << i << '\n'; };

        void operator()() const
        {
            if (void_func_ == nullptr) // specialized compare with nullptr
                default_func(7);
            else
                void_func_(7);
        }

    private:
        SomeVoidFunc void_func_{};
    };

    void user_func(int i)
    {
        std::cout << (i + 1) << '\n';
    }

    int main()
    {
        C c1;
        C c2(user_func);
        c1();
        c2();
    }

Output:

    7
    8

### See also

operator bool | checks if the `std::move_only_function` has a target (public member function)
operator==operator!= (removed in C++20) | compares a std::function with nullptr (function template)

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